1. Field of the Invention
The present invention relates to an improvement of a display matrix substrate used in a liquid crystal display apparatus for displaying an image, a production method of the display matrix substrate and a display matrix circuit used for the display matrix substrate.
2. Description of the Related Art
Generally, a liquid crystal has a mobility like a liquid while maintaining a certain molecular arrangement like a solid. This molecular arrangement is readily changed responsive to an intensity of an electric field applied thereto. The change of the molecular arrangement brings a change of the optical characteristics of the liquid crystal. As an apparatus employing such a liquid crystal, there is well known a liquid crystal image display apparatus (referred to as liquid crystal display apparatus).
In the liquid crystal display apparatus, the liquid crystal is enclosed between a common electrode and a matrix substrate having pixel electrodes disposed to face the common electrode, wherein the pixel electrodes can be individually controlled. The optical characteristic of the liquid crystal interposed between the respective electrodes and the common electrode is changed responsive to an intensity of electric field applied across the common electrode and each of the corresponding pixel electrodes by selectively applying a data signal to the respective pixel electrodes.
This type of the liquid crystal display apparatus is further classified into two types, a transmission type and a reflection one. The transmission type liquid crystal apparatus has a capability of reducing a production cost of the apparatus because of a simple structure thereof. On the other hand, it has a drawback of less brightness of a displayed image because downsizing a display pane of the apparatus brings an increase of a ratio of an area occupied by switching transistors for selectively driving the pixel electrodes and wirings, resulting in a decrease of a vignetting factor. Thus, the brightness of the displayed image is decreased.
On the other hand, in the reflection type liquid crystal apparatus as disclosed in the Japanese Patent Publication 57-39422 and Japanese Patent Laid-open Publication 4-338721, the switching transistors and the wirings can be provided under reflective pixel electrodes. Thus, it is possible to obtain a bright image without a decrease of the vignetting factor even when the display panel is downsized.
Accordingly, it is suitable for a magnifying projection type liquid crystal display apparatus to employ the reflection type display panel having small dimensions with a high density.
Next, a description is given of the reflection type liquid crystal display apparatus equipped with MOS type transistors a conceivable one of the liquid crystal display apparatuses.
FIG. 17 shows a block diagram of a circuit for operating pixels arranged in a matrix form in a conceivable liquid crystal display apparatus; and
FIG. 18 is a sectional view of a unit pixel of a display matrix substrate shown in FIG. 17.
In FIG. 17, a reference character 1 designates a switching element of a MOS transistor. A plurality of the switching elements 1 are vertically and horizontally disposed in matrix on a glass plate or a silicon substrate (not shown). A pixel electrode 2 and a capacitor 3 for storing electric charges, of which one of terminals is commonly used with the pixel electrode 2, are connected to a source 8 or a drain 7 of the switching element 1. In this case the pixel electrode 2 is connected to the source 8. A gate line Xi for selectively supplying a selection signal is connected to a gate electrode 4 of the switching element 1, and a signal line Yj for supplying a video signal is connected to either the drain 7 or the source 8 which is not connected to the pixel electrode 2. To the gate line Xi, the selection signal is applied from an X direction scanning circuit Xscn. And to the signal line Yj, the selection signal is applied from a Y direction scanning circuit Yscn.
To each of the pixel electrodes 2, there is provided a transparent common electrode (referred to as common electrode) 5 so as to face each of the pixel electrodes 2, and a liquid crystal 6 is enclosed between the common electrode 5 and the pixel electrodes 2. Thus, a pixel is formed with every pixel electrode 2.
Upon an operation of the switching element 1, when a selection signal is applied to the gate electrode 4 through the gate line Xi, the switching element 1 of the MOS transistor is turned on. Thus, the video signal passes the switching element 1 through the signal line Yj, and is simultaneously applied to the pixel electrode 2 and the capacitor 3, resulting is that the capacitor 3 is charged.
Here, an electrical potential of the pixel electrode 2 is maintained responsive to an amount of the electric charges stored in the corresponding capacitor 3 even when the selection signal from the gate line Xi becomes zero, i.e., in a non-selection mode.
During this time, the liquid crystal 6 undergoes an electrical potential applied across the pixel electrode 2 and the common electrode 5. Thereby, a light transmittance of the liquid crystal 6 is modulated. Thus, the electric signal (the video signal) can be converted to a modulated optical signal by controlling an intensity of the electrical potential applied across the pixel electrode 2 and the common electrode 5.
Accordingly, an image can be formed by disposing such unit pixels in matrix on a display panel and by vertically and horizontally scanning both the gate signal and the video signal. As a scanning method mentioned above, all the switching elements 1 along the gate line Xi arranged in an X direction are turned on. Thus, the video signal is applied to the pixel electrodes 2, and the capacitors 3 are simultaneously charged thereby. And the following gate lines Xi are successively turned on by being scanned with the gate signal in a Y direction.
Next, the description is given of a structure of the unit pixel, referring to FIG. 18.
The switching element 1 made of the MOS transistor as mentioned above, has the gate electrode 4, the drain 7, and the source 8. The gate electrode 4 is provided on a substrate 10 made of a single crystal silicon through a gate oxide layer 9. The gate electrodes 4 disposed in the X direction are formed as the gate line Xi by using, for instance, polycrystal silicon (FIG. 17). Further, the drain 7 is connected to the signal line Yj extending in the Y direction (FIG. 17).
Next to the source 8, the capacitor 3 for storing the electric charges is formed by causing an insulation layer made of Si0.sub.2 to be interposed between the substrate 10 and a capacitor electrode 11, and the capacitor electrode 11 is connected to the source 8. The pixel electrode 2 is formed, for instance, across an insulation layer 13 over the switching element 1 and the capacitor 3. The pixel electrode 2 and the source 8 are electrically connected through a cutout 14 (enclosed by a broken line in FIG. 18) defined in a thickness direction for an electrical connection.
The pixel electrode 2 is made of such a material as aluminum having a high reflectivity for a read light.
A transparent glass plate 16 coated with the transparent common electrode 5 thereon is provided over the surface of the display matrix substrate 15 formed with the various elements mentioned above so that the transparent common electrode 5 faces the surface of the display matrix substrate 15 leaving a little clearance L1 therebetween. On both surfaces of the transparent common electrode 5 and the pixel electrode 2, there are respectively provided alignment layers 17, 18, and the liquid crystal 6 is enclosed between the pixel electrodes 2 and the transparent common electrode 5, resulting in a liquid crystal panel. Thus, the reflection type liquid crystal display apparatus is produced. An incident light 19 coming from an upper direction of the transparent glass plate 16 passes through the liquid crystal 6, and is reflected by the surface of the pixel electrode 2 and outputted as a modulated light beam 20.
Incidentally, in order to obtain an excellent image quality, the apparatus required at least 300 thousand pixels (640.times.480 pixels) for a monochrome picture, and several thousand million pixels for a color. Thus, it is necessary to form transistors as the switching elements as many as the number of the pixels.
In a present semiconductor production process, a certain amount of the transistors would be inevitably rejected due to forming faults of the transistors when producing the several thousand million transistors.
In a development of a defective transistor in the display matrix substrate, there occurs a defect of a bright spot or a black spot, resulting in a degradation of display quality of the image. Presently, the production cost of the display matrix substrate having several million pixels is already very high. Thus, discarding a whole display matrix substrate containing such defective transistors incurs a further increase of the production cost. For this problem, there are proposed devices for correcting the defective pixels or making them inconspicuous.
As to such defective pixels, it is possible to detect a defective pixel by applying a driving signal and a video signal to the display matrix substrate, and synchronizing a current response of the video signal with the driving signal of the display matrix substrate after the transistors as the switching elements and the matrix lines made of the signal lines and the gate lines are completed.
As examples of correcting the defective pixels or making them inconspicuous, there are various methods as below.
(1) Method for coating a black color on a part of the glass substrate corresponding to defective pixels. PA1 (2) Method for removing a part of the liquid crystal corresponding to the defective pixels by utilizing optical energy or heat energy (Japanese Patent Publication 7-46181). PA1 (3) Method for providing inspection terminals in the substrate and fusing off a part corresponding to the substrate and fusing off a part corresponding to the defective pixels with a laser beam in response to the inspection result (Japanese Patent Publication 7-78673). PA1 (4) Method for fusing off scanning electrodes and signal input terminals of the transistors with a laser beam and connecting the defective pixels to the adjacent pixels with a laser beam (Japanese Patent Publication 4-39055).
However, in the various methods mentioned above, there are drawbacks as follows.
In the case where the defective pixel is coated with the black color as mentioned in the item (1), the black spot appears conspicuous in a bright image though it is not so in a dark image. In the case where the liquid crystal of the defective pixel is removed as mentioned in the item (2), it is difficult to remove the defective pixel when the area of the defective pixel is small. Further, the circumferential pixels of the defective pixel are subject to damage by heating. In the case of the melt-cutting by using the laser beam as mentioned in the items (3) and (4), corresponding pixels are tend to be damaged by heating, and the luminance of the corresponding pixels becomes unstable as their driving transistor fused off are no longer functioning.